Field of the Invention
This invention relates to the separation and enrichment of a predetermined isotope iQ of a multi-isotopic element Q (possessing isotopes iQ, iQ, kQ, etc.), and in particular to a process and apparatus for condensation repressing isotope separation by laser activation.
Description of Related Art
Since the discovery of isotopes around 1925, (70+ of the 92 known elements have multiple isotopes) various methods have been developed to separate these isotopes. Because the isotopes of a given element behave the same chemically, and differ only by one or more mass units due to different neutron populations in their nuclei, the standard chemical separation techniques employed for separating different elements cannot be used. Until recently, the predominant isotope separation methods utilized have employed diffusion and centrifuging, both of which take advantage of the small mass differences between isotopes. While many separated isotopes have become important in materials research and in nuclear medicine, the largest application of isotope separation has been that of separating and enriching uranium. These separated and enriched uranium isotopes fuel nuclear fission power plants, in particular the U-233 and U-235 isotopes of uranium, as well as Pu-239 bred from U-238. Although breeder reactors may ultimately also consume the 99.3% of U-238 in natural uranium, for the next fifty years, enriching the U-235 present in natural uranium, from 0.7% to about 5% as required in nuclear reactor fuels, will remain a multi-kiloton isotope processing operation worldwide.
While gas centrifuge enrichment of uranium hexafluoride (UF6) remains the primary enrichment process employed today (in 2013), after the discovery of the laser in 1960, a number of laboratories started to re-investigate the photon-induced separation of atomic uranium isotopes, a technique considered briefly during WWII, using the very narrow monochromatic spectral lines that the newly discovered lasers could provide. Such narrow monochromatic spectral lines allow for isotope-selective photon absorptions and excitations of isotopes, such as the isotopes of U-235 or U-238, due to small mass-dependent spectral isotope-shifts between different isotopes thereof. This approach to isotope separation promises larger isotope enrichments per stage than is possible with mass diffusion schemes. This research into laser photon isotope separation resulted in the expensive government-supported AVLIS and SILVA atomic-vapor enrichment programs in the USA, France, and elsewhere.
After the discovery, in the 1970s, of high-power CO2, CO, HF, and other molecular lasers, attention turned to the possibility of employing such lasers for less costly molecular laser isotope separation (MLIS) schemes using gaseous UF6 molecules instead of vaporized uranium atoms. Infrared molecular lasers can selectively excite vibrational energy levels in 235UF6 or 238UF6 isotopomers, either directly or via a wavelength-conversion medium. While atomic spectroscopy and lifetimes of electronic energy levels in uranium atoms were well understood, permitting rapid development of AVLIS programs, the rovibrational energy states and lifetimes of large molecules such as UF6 were less known, and required additional research to understand. Besides molecular spectroscopy, several new mechanisms had to be taken into account, such as rotational transitions, intra- and extra-molecular vibrational energy conversions and transfers (VT, VV), and van der Waals dimer formations.
In general, a LIS process comprises two inter-dependent steps: (1) laser excitation, and (2) isotope harvesting. After one finds a laser that can successfully excite selected isotopomers in a gas or gas mixture, means must be found to rapidly remove excited species from the mixture before they loose their excitation. In AVLIS, where electronic excited states can only “live” for microseconds, rapid (nanosecond) consecutive laser excitations are employed that take an isotope from the ground level to the ionization level in two or three successive excitation steps with successive isotope-selective absorption of two or three resonant laser photons. The selectively ionized isotopes are then collected and separated from unexcited species by an electromagnetic field. In MLIS, where selectively excited vibrational states can exist for milliseconds or longer (depending on gas collision rates), several schemes have been developed to separate excited from non-excited isotopomers. One harvesting method involves molecular obliterations (MOLIS), which uses rapid (sub-microsecond) successive multi-photon absorptions “up the vibrational ladder” in an isotopomer until the isotopomer reaches its dissociation limit. Selectively dissociated non-volatile isotopomer products then react to form solid particles that are separable from non-excited gaseous species. In a second MLIS harvesting scheme, acceleration of chemical reactions between selected isotopomers and a (slowly reacting) co-reactant gas RX is sought (CHEMLIS), using laser photons to induce isotope-selective multi-level vibrational excitations that promote atomic re-arrangements in attachment complexes UF6:RX. In this case, reaction products precipitate out or form solid particles, thereby permitting separation from unreacted gas.
In a third MLIS scheme, of greatest applicability to this application, dimer formation of iQF6 in a super-cooled supersonic free jet is selectively suppressed. The iQF6 gas is mixed as a minor component in a carrier gas G which has a favorable gas coefficient to provide rapid cooling of the supersonic free jet as it expands into a low-pressure evacuation chamber. Cooled-down selected iQF6 isotopomers in the jet are laser-excited, which inhibits their condensation in the free jet. This process has been dubbed CR-MLIS or CREMLIS for Condensation Repression Molecular Laser Isotope Separation. Non-laser-excited molecules will form QF6:G dimers and tend to stay in the jet core. The heavier QF6:G dimers may be separated with a skimmer from the previously laser-excited lighter QF6 monomers in the expanding free jet which radially flee out of the jet core in greater abundance. Because the flow is supersonic, there is no back-streaming of gas from the skimmer into the vacuum chamber. As it enters the skimmer, the gas flow goes through a standing shock in the skimmer's mouth as it returns from supersonic to subsonic flow. In the case of UF6, lighter laser-heated 235UF6 (or 238UF6) monomers are preferentially chased out of the central core region of a free supersonic jet, while heavier UF6:G dimers stay mostly in the core. The skimmer then separates the jet's core gas from the isotope-enriched monomer rim gases. In the UF6-specific laser enrichment processes known as SILARC or CRISLA and SILEX (SILARC=Separation of Isotopes by Laser Activated Repression of Condensation; CRISLA=Condensation Repression Isotope Separation by Laser Activation; and SILEX=Separation of Isotopes by Laser Excitation), the skimmer-caught dimers will dissociate again into gaseous monomers in the post-skimmer subsonic room-temperature collection chamber. This is due to the weakness of the van der Waals dimer bonds which last for only micro-seconds at room temperature, but which persist for milliseconds at low supersonic jet-cooled temperatures during the jet's typical 0.1 milliseconds of travel time through the vacuum chamber. The post-skimmer UF6:G→UF6+G dissociations allow continuous inter-stage cascade operations similar to those in Diffusion and Gas-Centrifuge uranium enrichment plants. On the other hand, in MOLIS or CHEMLIS processes, solid isotope-depleted or -enriched products are deposited on walls or scrubbers, which must be intermittently re-evaporated or re-fluorinated after a build-up of several particle layers. For this reason, CRISLA and SILEX selective dimerization repression or dimer association methods are much preferred in commercial applications.
Gaseous atoms and molecules when approaching each other are attracted to each other by so-called van der Waals forces which are proportional to the inverse sixth power of their separation. In collisions they would cling if it were not for the fact that they must shed their relative kinetic energy. At room temperature, most collisions therefore do not result in clinging (dimer formation), but cause a rebound after two species collide. However because the relative kinetic energies in collisions follow a Boltzmann distribution, a small number of low-energy collisions can result in adherence by conversion of the relative kinetic energy into rotational energy of the resulting dimer. Thus, a small fraction of dimers can form via 2-body collisions. At room temperature the dimer marriage is short-lived however and the dimers dissociate again after a few additional intermolecular collisions. At any instant the fraction of molecules in the dimer state is therefore small compared to the number of molecules in the monomer (=single molecule) state. However as the gas temperature is dropped, the fraction of low-energy collisions increases, and dimers live longer before they disassociate. At sufficiently low temperatures dimers are prevalent over monomers. In super-sonic self-cooling free jets as used in CRISLA and SILEX, cold dimers form and exist long enough to remain intact through-out the sub-millisecond long travel time from nozzle exit to skimmer entrance.
However, vibrationally laser-excited molecules are too energetic/hot to form dimers. Upon meeting a potential partner and trying to form a van der Waals dimer bond, such excited molecules release their vibrational energy in sub-microsecond timeframes. This energy is converted into kinetic energy that prevents the molecules from dimerizing and forces the molecules to recoil apart. Thus, isotope-selectively laser-excited molecules are prevented from forming dimers and tend to flee/escape the core of the free jet, thereby entering the rim or background stream in the evacuation chamber. Conversely, non-excited molecular species tend to form heavy dimers and tend to remain in the core of the jet, which is captured by a jet skimmer that pumps out the jet's core gas separately from the chamber background or the jet's rim gases.
It is advantageous to have the mass of the dimer partner molecules close to that of the isotopomer to be separated. For example for UF6/G gas mixtures, the closer the mass of the dimer-forming collision partner G is to the mass of UF6, the more kinetic energy a previously excited UF6 monomer acquires in de-exciting and recoiling from G, as occurs after an attempted dimerization event between UF6 and G. Due to these de-exciting and recoiling events the desired isotopomer of UF6, in addition to being generally lighter and faster than dimerized UF6:G, gains extra kinetic energy that helps it escape the jet core. Since super-cooling of UF6 molecules must take place in excess carrier gas to keep UF6 gaseous, a suitable dimer partner G would be inert xenon (Xe), which is the heaviest stable monatomic gas with the highest expansion coefficient γ=1.66, which contributes to quick adiabatic supersonic cooling, and cannot absorb vibrational infrared radiation. Relatively heavy and inert SF6 (with γ=1.3) is another possible choice, but care must be taken that it does not scavenge some of the infrared laser radiation intended to excite only UF6. The intermolecular van-der-Waals bond strength between UF6 and (expensive) Xe is also different from that between UF6 and (less expensive) SF6, but either of these or other suitable heavy partner gas can be utilized.
Cooling of a gaseous UF6 feed stream in an MLIS process is generally desirable because the fundamental (strongest) vibrational v3 absorption bands in the 16-micron wavelength region for heavy 235UF6 and 238UF6 molecules overlap considerably at room temperature. Although the Q-peak of the fundamental v3 vibration of 238UF6 around 627.7 cm−1 is isotope-shifted by 0.6 cm−1 to 628.3 cm−1 for 235UF6, at room temperature (−300 K), the P- and R-branch-broadened isotopic absorption bands overlap each other considerably. Only at lower temperatures do the spectral bands of 235UF6 and 238UF6 shrink and separate, allowing better selective laser excitation.
Unfortunately, the vapor pressure of UF6 at 243 K is only 1 Torr, and at lower temperatures it condenses into a solid. The solution is to laser-irradiate a mixture of gaseous UF6 diluted with a volatile carrier gas (G) in an adiabatically self-cooled supersonic jet, wherein the super-cooled UF6 molecules can remain gaseous during jet expansion. In this technique, one passes a UF6/G gas mixture from a feed tank through a supersonic nozzle into an evacuated low-pressure chamber where it forms a supersonic supercooled jet. This approach allows the UF6 to cool down, as a dispersed gas, to very low temperatures (below 100 K) for a very brief period (milliseconds) as it travels through the chamber, before it is captured by a jet core “skimmer” which acts as a diffuser that takes the supersonic gas stream back to subsonic flow and ambient temperatures. Besides pressure recovery and a return to subsonic flow, the skimmer also separates the expanded gas into two separated streams in which the relative 235UF6 and 238UF6 concentrations are different. This approach was used in MOLIS research, but problems arose when the supersonic gas mix was super-cooled too much, and UF6 started to dimerize to UF6:G and formed particles in the downstream jet. Instead of fighting dimerization, it was then conceived to take advantage of the dimer formation phenomenon, resulting in the present CREMLIS-based CRISLA process.
In summary, of the MLIS harvesting techniques, MOLIS and CHEMLIS have proven to be commercially unattractive because of smaller-than-expected separation factors and inter-stage chemical reprocessing requirements. CRISLA and SILEX have been found acceptable, primarily because no inter-stage reprocessing is needed, and single-stage enrichment factors of β≥2 are achievable, making these processes competitive with centrifuge enrichment. Using CRISLA, only three or four stages of enrichment are required to enrich natural uranium from 0.7% U-235 to 5% U-235, compared to the ten or more stages required in centrifuge separation techniques. Retrieval of UF6 in the product and tails streams of UF6/G mixtures, where G is a more volatile gas, can be achieved by simple cryo-trapping of UF6 in cold-traps as is well-known to those familiar with the art.
In a recently publicized SILEX uranium enrichment pilot program, pulsed 16-micron lasers were used to excite 238UF6 and 235UF6 molecules in a supersonic jet as described above. These 16-micron lasers were originally developed for MOLIS and CHEMLIS research programs in the 1970-1990 period, and comprise a pulsed high-power 10-micron CO2 laser and a liquid-nitrogen-cooled para-hydrogen-filled raman conversion cell that converts the 10-micron laser lines from a pulsed CO2 TEA laser to 16-micron laser lines. Except for this H2-raman-converting CO2 laser, an extensive three-decades-long worldwide search (see Eerkens, J., editor; “Laser Isotope Separation—Science and Technology”, Vol. MS 113, SPIE Optical Engineering Press) failed to find another suitable pulsed or continuous 16-micron laser that could produce fine-tuned laser frequencies at high laser power levels with adequate overlap of the Q-Branch absorption peaks of cold gaseous UF6. The most advanced of the pulsed 16-micron raman-converted CO2 laser systems was developed in South-Africa (SA), which has been adopted by the industrialized SILEX pilot program.
One problem that has been encountered with the SA 16-micron lasers is that the maximum pulse repetition rate (prr) of the driver CO2 TEA laser is limited to 500-1000 Hz at the desired operating wavelengths while approximately 10,000 Hz is desired, as reported in the literature. Because the 16-micron-laser-irradiated UF6/G gas mixture passes through the irradiation chambers at supersonic speeds in 50-100 microseconds, and because laser pulses last about 0.1 microseconds, there will be long dead-times (no laser-irradiation) between pulses for periods of 1000 microseconds if the pulse repetition rate equals only 1000 Hz. The average laser-irradiation duty factor is then only 10% unless the prr can be raised. This problem may be partly solved by pulsing feed gas flows to coincide with laser irradiation periods. Alternatively one could laser-irradiate the process gas with ten separate 1000 Hz lasers whose outputs are time-multiplexed. Such measures would, however, increase the system's complexity and process costs considerably.
Another problem with the H2-raman-conversion CO2 laser is that laser spectral frequencies require additional super-fine microwave-shifted tuning to optimize absorptions by 235UF6 or 238UF6. Together with inadequate 1000 Hz pulse repetition rates and the inherently low (−0.2%) electricity-to-laser energy conversion efficiencies of raman-converted 16-micron CO2 lasers, these problems have made the SILEX process with fine-tuned pulsed CO2 lasers more expensive than centrifuge enrichment of UF6. In addition there are 16-micron-laser losses due to Dicke super-radiance from intense short laser excitation pulses that reduce the laser efficiency even more.
It is thus a first object of the present application to present an advanced CRISLA process and system that is less expensive and less complex than presently known SILEX and CREMLIS processes.
It is a further object of the present application to present an advanced CRISLA process and system that allows for high percentage enrichment of a selected isotopomer species within a single separation step.
It is a further object of the present application to present an advanced CRISLA process and system that employs a novel laser excitation methodology that prevents dimerization of a desired isotopomer, and promotes its expulsion from a free jet thereby separating the desired isotopomer from a mixture of isotopomers.
It is a further object of the present application to present an advanced CRISLA process and system that employs a simple, efficient, inexpensive, and robust CO laser, instead of an inefficient and expensive CO2 laser and raman conversion cell.
It is a further object of the present application to present an advanced CRISLA process and system that employs intracavity laser excitations of target molecules with a minimum of optical windows and high bidirectional laser power flux.
It is a final object of the present application to present an advanced CRISLA process and system that may be designed and operated in massively serial or parallel configurations with only a single CO laser.